Recently, an internal temperature of the electronic equipment is increased as a heating value (HV) in electronics parts and circuits increases with production of small electronic equipment with high performance and high functionality, which leads to problems such as erroneous operation of semiconductor devices, undesirable changes in characteristics of resistor parts and shortened life span of their parts. Various techniques have been used as a heat radiation method for solving these problems. Heat sinks, fans and pipes have been used for the heat radiation, but the recent use of excellent heat-dissipating materials gives various significant advantages and effects.
From the abovementioned point of view, there have been attempts to endow a steel sheet with heat-absorbing and/or heat-dissipating properties. That is, there is an attempt to give heat-absorbing and/or heat-dissipating properties to a steel sheet by mixing a pigment such as carbon black and titania with polymeric resin to form a coating layer on the steel sheet, the pigment having excellent heat emissivity in an infrared wavelength range. When these methods apply to electronic equipment using a box-type steel sheet, it is possible to effectively radiate internal heat from the electronic equipment.
In the use of these methods, a pigment should be present in large quantity to give excellent heat-absorbing and/or heat-dissipating properties. As a result, the manufacturing cost is increased and the electrical resistance is high since a coating layer is thick in thickness. Electrostatic earth properties are required to prevent an electromagnetic wave from being generated in the electronic equipment, and therefore a steel sheet should have good surface electrical conductivity.
To manufacture a steel sheet having surface electrical conductivity and electromagnetic wave shielding property, a method has also been used in recent years, including: adding expensive metal powder such as Ni and Cu to a resin composition for surface-treating a steel sheet. However, this technique has disadvantages that the manufacturing cost is high and scratches are formed during the press finishing process. Also, there has been recently proposed a black resin coated steel sheet prepared by subjecting a surface of a Zn—Ni steel sheet to the black oxidation and coating the steel sheet with a resin composition, but this method has problems that it is difficult to manufacture a steel sheet whose surface is good and smooth and the steel sheet has its limit to give corrosion resistance and excellent surface conductivity.